Williamson County Septic Rules: The Adopted Thresholds

For most of a year the figures below were withheld from this site, because the only document we could get to load was marked revision 3 — DRAFT, and publishing a threshold a homeowner might design a lot around from a draft is worse than publishing nothing. The adopted regulations turned out to load after all — 436 pages, adopted May 2000, amendments through March 2025 effective July 2025 — and every figure in the draft matched. What follows is from the adopted text, with the section each comes from.

Who runs it

Williamson is one of nine Tennessee counties that operate their own on-site sewage program under contract with the state, and has since January 1993. The agency is the Williamson County Department of Sewage Disposal Management, 1320 West Main Street, Suite 411, Franklin, (615) 790-5751. The state's Rule 0400-48-01 — the one that governs Maury County next door — is not the operative document here; the county's own regulations are, and where the two differ, the county's control.

Two practical consequences. An installer needs the state permit and the county's licence. And every system needs the county's separate Permit to Install, issued per installer per system, which voids if a layout inspection is not requested within 45 days.

The four thresholds

Absorption rate

Section 19(A) sets a window with a floor and a ceiling, and both matter. Soils with an absorption rate under 15 MPI — "between 1MPI and 14MPI" — "shall not be utilized for subsurface sewage disposal system installations": fast soil does not filter. Soils rated "15MPI, 30MPI, 45MPI, 60MPI and 75MPI may be utilized for conventional system use." Soils "in excess of 75MPI shall not be approved for conventional system use," though the regulation goes on to say such sites may be assessed for other designs. The rate is determined "by conducting a detailed field examination of the actual soil characteristics and properties," using the county's soil series and rating criteria in its Appendix 1 — which is to say by a soil professional, not a bucket of water.

Soil depth

"There shall exist a minimum of thirty-six (36) inches of natural soil over any layer(s) of bed rock or the upper limits of any type of water table." Around Nolensville the federal soil survey maps Talbott soils at 32 inches to bedrock and 15% of the ground within four miles as bare rock outcrop; College Grove and Arrington are led by the same constraint. The rule and the ground are the reason so many Williamson permits end up as alternative designs.

Lot size

"A land parcel shall be a minimum of one (1.00) acre in size to be considered for the use of conventional subsurface sewage disposal systems." Compare the state's 20,000 square feet on public water in Maury — a Williamson conventional lot is more than twice the size.

Fees

Section 33, in full: "Fees shall be set for various Department services by the Williamson County Board of Health. The assessed fees for said Department services shall be listed and displayed at the Department offices." That is the regulation saying there is no schedule to publish. We have not invented one, and we would treat any site that quotes a Williamson County permit fee with suspicion.

Recertification, the county's inspection

Where the state offers a $200 inspection letter, Williamson offers recertification: an on-site investigation by a department representative to determine whether an existing system is failing. Its fee falls under Section 33 like everything else. What buyers should order, and what the state-side equivalent costs, is in what a septic inspection costs.

Spring Hill, and which side of the line you are on

Spring Hill straddles the Maury–Williamson line, and septic follows the county your parcel is in, not your mailing address. A lot on the Williamson side is on the rules above; a lot on the Maury side is on the state's. The difference is not academic: one acre against 20,000 square feet, a county licence against none, an unpublished fee against a published one.

The source

Everything above is from the county's Regulations Governing On-Site Sewage Disposal Systems, adopted 16 May 2000 with amendments through 18 March 2025, effective 1 July 2025. The excerpts this site relies on are kept alongside the site's data so the quotes can be checked against the original. If the county amends again, the section numbers above are where to look.